Protocol for the examination of placentas of monozygotic twins, to include those of the TTTS
K. Benirschke June 16, 2000

It is difficult to examine the placenta of the transfusion
syndrome for many reasons. In the first place, it is often
disrupted and this makes injection study difficult. Moreover,
these placentas often are from premature deliveries with placentas
that are normally more delicate. Most important is that the
examiner is familiar with the distribution of blood vessels
in the normal placenta so that he/she can assess the sometimes
very hard-to-see anastomoses that are the basis for the transfusion
syndrome. Thus he/she should become familiarized by examining
normal singleton placentas beforehand.

The transfusion syndrome happens essentially
ONLY in monochorionic twin placentas. They may be monoamnionic
(MoMo) or diamnionic (DiMo). One of the first requirements
is that this status is assured. Thus, section of the “dividing
membranes: of DiMo placentas must be made for the record.

PROCEDURE:At delivery, the umbilical cords of twins (A,B) need
to be labeled with one or two clamps and this needs to be
recorded and compared with their birth weights. Next, the
lengths of the cords are measured (all of it) and recorded.
Next, the insertion site of the cords must be documented (central,
eccentric, marginal, velamentous). The number of blood vessels
in e cord needs to be recorded, then the cords should be detached
from the placental mass. The “dividing membranes”,
if present, needs to be sampled for histology. The peripheral
membranes should then be trimmed and then the placenta is
weighed and measured. Then the maternal surface should be
inspected for 1) completeness and/or disruption (it helps
to know for the later purpose of injection), and 2? Color.
It is especially relevant to note if both fetuses’ halves
have the same darkness/lightness because this reflects the
amount of fetal hemoglobin (thus, this is an indication of
anemia and/or plethora).

Next is the fetal surface, the most critical
part. The examiner should assess roughly what portion of placental
mass is perfused by each twin. Is it 50:50 or is it 80:20,
etc. This can be estimated by identifying the “placental
vascular equator”. When one pursues, visually, the distribution
of blood vessels as they emanate from the insertion of the
cords to the periphery the “equator” becomes obvious.
At their ends, the fetal blood vessels dip into the villous
tissue where they then perfuse a fetal cotyledon. Arteries
normally cross over the veins and are thus easily identified
(especially in their large calibers near the cords). In the
normal placenta, an artery leads to such a peripheral cotyledon,
dips down, perfuses the cotyledon, and a corresponding vein
brings the blood back from this cotyledon to the same fetus.
One needs to identify BOTH these vessels and assure oneself
that they come and go to/from the same umbilical cord.

The commonest communication between two
monochorionic twins is a single artery-to-artery (A-A) connection
on the surface of the placenta; these do NOT dip into villous
tissue but are major shunts between the twins and blood can
shift back and forth from one twin to the other, depending
on blood pressure, heart rate, etc. A vein-to-vein direct
anastomosis is uncommon. The A-A anastomosis is of variable
size, often it is quite large. But these communications may
also be so small that they are hard to detect. It often helps
to push blood gently back and forth in the surface vessels
and thus make visible small anastomoses, but at times it requires
injection.

THE MOST
important communication that is the major responsible agent
for the development of TTTS is one where at lest one cotyledon
is shared. That is to say, an artery comes from one twin (the
donor), dips into a cotyledon, which is then drained by a
vein into the other fetus (the recipient), who gets to be
larger and plethoric. VERY careful inspection of the fetal
surface can actually identify these “common villous
districts” (the shared cotyledons). At times though
it requires injection. What’s worse, such shared cotyledons
may be multiple, of different size, and they may have different
blood flow directions. Thus, I always draw a picture of the
surface blood vessels and/or photograph them. When I am uncertain
or want to demonstrate the communications, then I inject them
with milk.

INJECTIONIt is foolhardy to try to inject the entire placenta
as it is most commonly disrupted somewhere and this defect
allows fluid to drain and thus prevents cotyledonary filling.
I use milk because it is readily available, can be easily
seen when the vessels and villi are filled, and it does not
destroy the tissue for later histologic study. I use a completely
filled 20 ml syringe with an 18 gauge needle. Then I identify
the potential major A-A region or shared cotyledon, so identified
from inspection alone, choose it and try to inject this cotyledon
only. The nearest feeding (or draining) vessels of adequate
size is chosen for injection. One must be careful to insert
the needle almost horizontally to the placental surface so
as to prevent it from being double punctured. I shove the
needle in the chosen vessel for some distance, occlude the
vessel around the needle with my fingers and then slowly inject
as much milk as is needed to fill the entire cotyledon, until
the blood/milk mixture emanates from the other vessel that
leads to this shared cotyledon. (If later one inspects that
maternal surface or, later still, sections this region, one
finds this one cotyledon to be white, as all the blood has
been replaced by milk). I record the findings carefully, best
by drawings. It is these vessels (actually only one of them)
that needs laser occlusion in the TTTS therapy.

A problem arises when one twin has died
prior to the twin delivery. When the death has been recent,
the anastomoses may still be visible or even injectable. But,
when fetal demise has occurred a long time prior to delivery,
then injection is fruitless and only very diligent inspection
may identify the possible offending blood vessel connections.
This may be true even when it is a large blood vessel through
which one twin may have bled into the dead twin; they can
be most difficult to identify.

MICROSCOPYRoutine sections can be made, but they rarely add
to unraveling the syndrome. True, the donor twin’s villi
usually are much smaller and those of the recipient are larger
and congested; rarely on may find a thrombus, but these findings
are of minor consequence; Perhaps the most useful histology
comes from a study of the amnionic surface, as the “stuck”
twin (donor) often has amnion nodosum, not present in the
recipient.